This invention relates to the preparation of aromatic isocyanates from aromatic urethanes by phenolic transesterification and thermal decomposition.
Organic isocyanates are useful intermediates in the preparation of urethane foams, coatings and fibers, as well as in the preparation of various phytotoxic and insecticidal agents. Aromatic isocyanates, such as toluene diisocyanate, are particularly useful and have gained widespread industrial recognition.
Heretofore, aromatic isocyanates have generally been prepared by the reaction of an amine in an inert solvent with phosgene. For example, U.S. Pat. No. 2,311,046, granted Feb. 16, 1943, describes the manufacture of aromatic isocyanates by reacting an aromatic amine with phosgene; and U.S. Pat. No. 2,689,861, granted Sept. 21, 1954, describes the manufacture of aromatic isocyanate by reacting an amine with phosgene in the presence of tetramethyl urea. Such processes, while satisfactory, do not proceed as straightforwardly as appears. The isocyanate which is formed may react with the starting amine to form substituted ureas, thus decreasing the yield of desired product. U.S. Pat. No. 2,683,160, granted July 6, 1954, discloses a variety of proposed improvements in the phosgenation process. However, even these improved processes are disadvantageous in many respects, principally in the use of phosgene and the production of acid by-products.
Methods have been proposed which eliminate the use of phosgene, but none have found commercial acceptance due to the cost of materials, unusual reaction conditions, poor yields, or unpredictable side reactions. Phosgene-free processes, in general, have involved the catalytic reaction of an organic nitro compound and carbon monoxide. U.S. Pats. Nos. 3,728,370, granted Apr. 17, 1973; 3,743,664, granted July 3, 1973; 3,600,419, granted Aug. 17, 1971; 3,632,827, granted Jan. 4, 1972; 3,597,466, granted Aug. 3, 1971, 3,585,231, granted June 15, 1971, 3,576,836, granted Apr. 27, 1971; 3,576,835, granted Apr. 27, 1971; 3,523,963, granted Aug. 11, 1970; 3,523,964, granted Aug. 11,1970; and 3,523,965, granted Aug. 11, 1970, disclose various noble metal-catalyzed reactions of aromatic nitro compounds with carbon monoxide to form an aromatic isocyanate.
Recent interest has been focused on the non-catalytic pyrolysis of urethanes to prepare isocyanates. For example, U.S. Pat. No. 3,734,941, granted May 22, 1973, granted May 22, 1973, describes the pyrolysis of urethane at a temperature of 400.degree. to 600.degree. C to prepare the corresponding isocyanate. Similarly, U.S. Pat. No. 3,870,739 describes the pyrolysis of urethanes at a temperature of 350.degree. to 550.degree. C under conditions of controlled residence time and pressure to form an isocyanate.
The harsh reaction conditions required and relatively low yield obtained by current phosgene-free isocyanate syntheses make it desirable to provide a synthetic route to aromatic isocyanates which does not employ phosgene, expensive reactants or catalysts, or harsh reaction conditions.